Various proposals have evolved for covering air bag units. One primary objective is to provide a cover having a weakened section that will separate to provide an opening through which the air bag can escape for deployment.
Air bags are typically mounted in canisters in the steering wheel or in an instrument panel or crash pad on the driver's side of the vehicle. If the air bag is mounted on the passenger side, the canister is typically stowed behind an instrument panel either in a top mount position or in a front mount position. Such mounting arrangements require that the cover material of the steering wheel hub or the skin or cover of the instrument panel be specially configured during manufacture to produce a weakened section to define a seam therein that will readily open with bag deployment.
Such manufacturing methods and resultant structure call for various functional and aesthetic requirements. For example, the seam must be configured to separate such that the air bag can be deployed immediately after vehicle impact. However, the cover must have a seam that is configured such that the air bag unit is protected against either accidental or intentional tampering and additionally the seam should be configured to meet long term weathering and interior design appearance requirements. As to design appearance, there is a desire in some cases to form the surface of the cover as a smooth uninterrupted surface so that there is no indication that there is an air bag unit disposed beneath the surface. This interior design objective is desired from both a human psychology standpoint and a pure esthetics standpoint. In the case of the psychological considerations, the smooth surface effectively meets the adage of "out of sight out of mind" such that a passenger is not reminded of the possibility of deployment of an air bag unit. Then there is the interior design consideration for smooth flowing surfaces to be incorporated in the cover material of an instrument panel on the passenger side.
U.S. Pat. No. 3,640,546, issued Feb. 1, 1972 to D. S. Brawn, includes a cover for an air bag unit having two doors that are covered by an outer layer of material that is weakened along a joint line by reducing the thickness of the outer layer. Such thin sectioned seam lines can cause a resultant depression over time that will essentially outline the outlet from the canister for the air bag component of the air bag unit. The arrangement does not solve either the psychological awareness problem or the problem of retaining a smooth, flowing surface appearance.
U.S. Pat. No. 4,246,213, issued Jan. 20, 1981 to Y. Takamatsu et al., discloses a method for casting a V-shaped section at the tear seam. While the outer surface of the cover material is smooth, the V-shaped configuration also reduces the thickness of the outer layer in an undesirable manner.
U.S. Pat. No. 5,082,310 discloses a cover for an air bag unit having two doors that are hinged for pivotal movement during air bag deployment. The doors are covered by a layer of foam and the layer of foam is covered by an outer skin having a V-shape groove that is cut to weaken the skin and again in an undesirable manner.
One problem common to designs that cut or score a tear seam into an outer skin is that plastic materials, especially at cold temperatures, are prone to becoming brittle such that the cut or score line can induce highly undesirable cracking along the seam. At opposite temperature extremes, typical skin cover material such as thermoplastic olefins or thermoplastic polyvinyl chloride become relatively soft and deformable such that they will stretch and tend to remain unbroken during initial bag deployment. Another problem that can arise with cut or scored tear seams is that they become observable through the outer surface following extended periods of exposure to heat and sunlight.
Another specific disadvantage of the cover shown in the above '310 patent and another cover shown in U.S. Pat. No. 4,886,630 is that they require that the outer skin be separately formed on a heated mold and then removed from the mold and cut before being co-molded to a foam layer in a subsequent molding operation.
The problems arising from separately forming a skin and then cutting the grooves therein are obviated by the present method that employs casting techniques disclosed in U.S. Pat. Nos. 4,623,503, issued Nov. 18, 1986 to E. Anestis et al., U.S. Pat. No. 4,562,025, issued Dec. 31, 1985 to John D. Gray, U.S. Pat. No. 4,925,151, issued May 15, 1990 to John D. Gray, and U.S. Pat. No. 5,288,103, issued Feb. 22, 1994 to Parker et al., all of which are commonly assigned to the assignee of the present invention and each of which is hereby incorporated by reference.